Distinguishing Enantiomers

The value of embodies the conformation-independent 3D arrangement of the atoms of the ligands of a chirality center in distance space and thus cannot distinguish between enantiomers. This distinction is introduced by the descriptor S , , . 

Although the usual absorption and scattering spectroscopies caimot distinguish enantiomers, certain techniques are sensitive to optical activity in chiral molecules. These include optical rotatory dispersion (ORD), the rotation by the sample of the plane of linearly polari2ed light, used in simple polarimeters and circular dichroism (CD), the differential absorption of circularly polari2ed light. 

It is convenient to distinguish between enantiomers by prefixing the sign of rotation to the nfflne of the substance. For exanple, we refer to one of the enantiomers of 2-butanol as (-l-)-2-butanol and the other as (—)-2-butanol. Optically pure (-l-)-2-butanol has a specific rotation [a]o of +13.5° optically pure (—)-2-butanol has an exactly opposite specific rotation [a]o of —13.5°. 

For the separation of racemic mixtures, two basic types of membrane processes can be distinguished a direct separation using an enantioselective membrane, or separation in which a nonselective membrane assists an enantioselective process [5]. The most direct method is to apply enantioselective membranes, thus allowing selective transport of one of the enantiomers of a racemic mixture. These membranes can either be a dense polymer or a liquid. In the latter case, the membrane liquid can be chiral, or may contain a chiral additive (carrier). Nonselective membranes can also... 

The reactions of allylboronates 1 (R = H or CH3) may proceed either by way of transition state 3, in which the a-substituent X adopts an axial position, or 4 in which X occupies an equatorial position. These two pathways are easily distinguished since 3 provides 7 with a Z-olefin, whereas 4 provides 8 with an E-olefinic linkage. There is also a second fundamental stereochemical difference between these two transition states 7 and 8 are heterochirally related from reactions in which 1 is not racemic. That is, 7 and 8 arc enantiomers once the stereochemistry-associated with the double bond is destroyed. Thus, the selectivity for reaction by way of 3 in preference to 4, or via 6 in preference to 5 in reactions of a-subsliluted (Z)-2-butenylboronate 2, is an important factor that determines the suitability of these reagents for applications in enantioselective or acyclic diastereoselective synthesis. 

Imagine that the two brothers are twins. They are identical in every way except one. One of them has a mole on his right cheek, and the other has a mole on his left cheek. This allows you to distinguish them from each other. They are mirror images of each other, but they don t look exactly the same (one cannot be superimposed on top of the other). It is very important to be able to see the relationship between different compounds. It is important to be able to draw enantiomers. Later in the course, you will see reactions where a stereocenter is created and both enantiomers are formed. To predict the products, you must be able to draw both enantiomers. In this section, we will see how to draw enantiomers. 

Stereo isomers have the same constitution, but a different spatial arrangement of their atoms they differ in their configuration. Two cases have to be distinguished geometric isomers (diastereomers) and enantiomers. 

Consider ethanol (key sentence coming up). If you were to replace each of the methylene protons in turn with some other group, Z, you would end up with a pair of enantiomers. We call this, the Z test. For this reason, the protons (or whatever groups may be involved, in molecules of the type X-CA2-Y) are described as enantiotopic. This is of no consequence in the spectrometer, because as we have mentioned, enantiomers are not distinguishable by NMR under normal conditions. 

We have seen that the spectra of enantiomers, acquired under normal conditions, are identical. The NMR spectrometer does not differentiate between optically pure samples, and racemic ones. The wording is carefully chosen, particularly normal conditions , because it is often possible to distinguish enantiomers, by running their spectra in abnormal conditions - in the presence of a chiral resolving agent. Perhaps the best known of these is (-)2,2,2,trifluoro-l-(9-anthryl) ethanol, abbreviated understandably to TFAE. (W.H. Pirkle and D.J. Hoover, Top. Stereochem., 1982,13, 263). Structure 7.4 shows its structure. 

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